Oil and gas operators often drill wells into unconsolidated sandstone reservoirs. To obtain high production rates from such reservoirs requires some means of filtering sand out of the fluid as it is drawn from the reservoir. Operators have used screens and gravel packing to accomplish this filtering procedure.
Screens may be used as filters by sizing the screen to block the flow of particles larger than a given size. Traditionally, a sieve analysis is performed on the formation sand prior to completion of the well and the formation sand particle size range is determined. A filter screen size is chosen which will block the largest fifty percent of the formation sand particle sizes.
However, using filters of this size has several problems. First, the available inflow area for these filters is typically only three to six percent of the total filter surface area, which greatly limits the maximum flow rate available from the well. Second, screens used in these filters are typically square or rectangular grids, so that when a particle plugs a grid opening, the inflow area of that opening is lost, further reducing the inflow rate. This lack of alternative flow paths results in a filter which plugs very quickly. Third, these filters are subject to high erosion rates because the fluid flow is effectively straight through the filter material.
One alternative sizing arrangement in these types of filters is to size the filter to block only the largest ten percent of the formation sand particle sizes. This sizing results in filter openings that are approximately twice as large as those which block the largest fifty percent of particle sizes. This sizing technique helps to alleviate the rapid plugging problems of the smaller openings, but also results in much higher erosion rates.
Gravel packing is accomplished by placing a screen or slotted liner in the wellbore production zone, then filling the annular area between the screen or slotted liner and the formation with a specially sized, highly permeable sand. The gravel pack sand is sized so that it will not flow into the production equipment string. It also blocks the flow of formation sand into the production equipment string. Although effective in reducing sand production, gravel packing can be time-consuming. It can also be difficult to accomplish in horizontal wells because of the plugging tendency of current filter screens.
It is a goal of this invention to provide an economical filter that has an open flow area of greater than thirty percent of its total surface area.
It is a further goal of this invention to provide a filter which is damage resistant and which has properties similar to API standards for tensile strength and collapse.
It is another goal of this invention to provide a filter with high erosion resistance in both flow and circulation conditions.
It is another goal of this invention to provide a filter which is highly resistive to plugging by sand or other particulate matter.